Prevent overflows in time step calculations with RT

In certain cases, overflows may occur when computing the hydro time step sizes in the timestep tasks. Specifically, this may occur when a) the total physical simulation time to be covered is short; b) there are particles with long time step sizes (e.g. like in voids); and c) a large number of sub-cycles is permitted.

I added a check and a fix for that.

I also noticed that I'm not properly initializing the RT time step data anywhere. It shouldn't be a problem, but I added it during first_init anyway. Better safe than sorry.